1. Field of the Invention
The present invention relates to organic transistor arrays, display devices and methods of fabricating display devices.
2. Description of the Related Art
Recently, there is active research related to organic thin film transistors (or TFTs) using an organic material. The organic thin film transistors have the following features. First, the flexibility with which the materials, fabricating method and product configuration usable by the organic thin film transistor is high. Second, the organic thin film transistor can be arranged in a large area. Third, the organic thin film transistor has a simple layer structure and can be fabricated by a simple fabricating process. Fourth, the organic thin film transistor can be fabricated by an inexpensive fabricating apparatus.
An electrode, an insulator layer, an organic semiconductor layer and the like forming the organic thin film transistor may be formed by methods such as printing, spin-coating and dipping. Hence, the organic thin film transistor can be fabricated at a cost considerably lower than that of a conventional thin film transistor using a semiconductor material such as silicon.
A display device can be obtained by fabricating an organic thin film transistor array in which the organic thin film transistors are integrated, and driving display elements using the organic thin film transistor array. Such a display device has a newly added value including the characteristics of the organic thin film transistors. For example, T. Okubo et al., “10.5-in. VGA All-printed Flexible organic TFT Backplane for Electrophoretic Displays”, AMD5-4L, IDW '07, pp. 463-464 proposes a flexible display device that is fabricated by combining an organic thin film transistor array fabricated by the printing process with an electrophoretic element. Such a flexible display device is suited for a display panel on a curved wall or a portable display, and is uneasily damaged when dropped. In addition, the fabrication cost can be suppressed by employing the printing process.
In order to improve the display quality of the display device described above, it is important to improve the pixel density. It is necessary to improve the integration density of the organic thin film transistor array that is used to drive the display elements. When integrating the organic thin film transistor array, it is necessary to pattern electrodes and organic semiconductor layers. For this reason, in order to improve the display quality of the display device, it is necessary to develop a patterning technique which can finely pattern the electrodes and the organic semiconductor layers forming the organic thin film transistor array.
Generally, the most simple conventional organic thin film transistor array has signal lines and selection lines that are perpendicular to each other, and a section of a single pixel including the organic thin film transistor has a rectangular shape. However, the existing organic thin film transistor array does not have a sufficiently high integration density because the patterning accuracy of the printing process is poor. For example, when patterning the electrode, the organic thin film transistor may malfunction due to a short-circuiting of the electrodes if the patterning accuracy of the printing process is poor.
On the other hand, a Japanese Laid-Open Patent Publication No. 2005-310962 proposes a method of fabricating a stacked structure, including forming a wettability varying layer that includes a material whose critical surface tension varies when energy is applied thereto, forming portions having different critical surface tensions including a low surface energy portion having a low critical surface tension and a high surface energy portion having a high critical surface tension by applying energy to a portion of the wettability varying layer, forming a conductor layer on the high surface energy portion by applying a liquid that includes a conductor material on the surface of the wettability varying layer, and forming an organic semiconductor layer on the wettability varying layer.
However, when patterning the organic semiconductor layer, for example, the organic semiconductor layer is formed at a portion other than a channel portion of the organic thin film transistor if the patterning accuracy of the printing process is poor. As a result, the organic thin film transistor may malfunction due to a deterioration in the contrast ratio caused by an increase in an off leak current or a crosstalk between the adjacent organic thin film transistors. The off leak current refers to the current that leaks even when the circuit or device is in an off state or standby state.
On the other hand, the Japanese Laid-Open Patent Publication No. 2007-36259 proposes a method of fabricating the organic thin film transistor by forming a barrier around the channel portion and patterning the organic semiconductor layer. But according to this proposed method, the number of fabrication processes increases, to thereby deteriorate the throughput and increase the fabrication cost.
Known patterning methods include the ink jet method and the dispenser method. The ink jet method and the dispenser method directly plot the patterns, and thus, these methods can considerably improve the material utilization efficiency. When the organic semiconductor layer is formed by the ink jet method or the dispenser method, it may be possible to simplify the fabrication process, improve the yield, and reduce the fabrication cost. When a polymer material soluble in an organic solvent is used as the organic semiconductor material, it is possible to adjust the solution (organic semiconductor ink) of the organic semiconductor material, and thus, the patterns of the organic semiconductor layer can be formed by the ink jet method.
However, the spreading of the ink drop or, the inconsistent landing of the ink drops of the organic semiconductor ink on a target surface may cause problems. As the integration density of the organic thin film transistor improves, the ink drop forming density increases, and the ink drop is greatly affected by the solvent atmosphere after the ink drop lands on the target surface, thereby making the ink drop drying time longer. That is, the ink drop of the organic semiconductor ink after landing on the target surface more easily spreads as the integration density of the organic thin film transistor improves. As a result, it becomes difficult to control the patterns of the organic semiconductor layer.
In addition, when the electrode is printed using the ink jet method using Ag ink, for example, there is a limit in reducing the pattern size if the printing accuracy is taken into consideration. Hence, the organic thin film transistor and the organic thin film transistor array in which the organic thin film transistors are integrated need to be formed by taking into consideration the spreading of the ink and the inconsistent landing of the ink drops on the target surface, but it is difficult to reduce the pattern size while maintaining the required printing accuracy.
Therefore, although various techniques have been proposed with respect to the organic thin film transistors (TFTs), there has yet to be proposed a technique that can improve the integration density of the organic thin film transistor array employing the existing printing process without increasing the number of fabrication processes.